This invention relates to collimators, and more specifically, to targets or reticles therefor.
In the field of optics, collimators are adapted not only for testing, calibrating and measuring instruments as described in U.S. Pat. Nos. 4,422,758, 4,022,533, 3,966,328, 3,915,575, 3,518,005 and IEEE Transactions on Instrument and Measurement, Vol. TARGET-34, No. 1, Mar. 1985, at page 51, but for target projection and simulation. Their use in calibrating and measuring instruments is attributable to their proficiency in producing parallel light rays. The use of collimators in target simulation is based on their far-field response. Placing a target in the focal plane of a collimator is a known technique for making a uniform target appear to be in the far-field. The image is said to be formed at infinity. The technique is to use standard targets which are commonly available as target sets. Such sets include three-bar and area targets. Three-bar targets are plates having equally spaced bars or slits, each about 0.32 cm. wide. Area targets are square aperture plates.
Standard targets, although in wide use, are nevertheless subject to certain disadvantages, particularly when the light source is not in the human visible range. Radiation at both ends of the spectrum is frequently used in collimators employed in instrument callibrating and target training. However, standard bar targets used with this radiation are subject to mechanical constraints. Since, like stencils, they are dependent upon cut-outs, images formed are subject to the same limitations as stencils. That is, bands or strips must support the cut-out image. Where the radiation entering the collimator is in the infrared range the targets are etched or otherwise cut out of metal foil. Mechanical constraints limit the size, area and precision of the target. In addition, the thermal effects of infrared absorption cause the noncutout portions of the target to undergo a temperature rise or drop. This results in a bleed-thru of the thermal radiation, and a lowering of the observed temperature change (delta T.degree.) in the object plane. The longer the target is exposed to IR energy the greater this error will become.
By the practice of this invention, mechanical constraints of cut-outs in targets now employed are obviated, and many of the thermal limitations of such targets are alleviated.